The present invention relates to a process for preparing trimethylol compounds and formic acid by reaction of formaldehyde and aldehydes in the presence of a nitrogen base followed by distillation of the resulting reaction mixture in the presence of an auxiliary.
Trimethylol compounds are widely used in the plastics sector for producing surface coatings, urethanes, and polyesters. Important trimethylol compounds are, for example, trimethylolethane and trimethylolbutane, but especially trimethylolpropane.
The industrial preparation of trimethylolpropane (TMP) starts out from n-butyraldehyde and formaldehyde that are reacted in a two-stage reaction process. In a first reaction step, 2,2-dimethylolbutanal is formed first in a base-catalyzed aldol condensation via the intermediate 2-methylolbutanal. In a subsequent cross Cannizzaro reaction, trimethylol-propane together with formate salts are formed in the presence of stoichiometric amounts of a base.
As base, use is usually made of inorganic compounds such as sodium hydroxide or calcium hydroxide. If calcium hydroxide is used as base in the process, the calcium formate obtained as coproduct can, for example, be used further for producing various animal fodder products and additives for animal fodder products. However, the sodium formate formed when sodium hydroxide is used is less desirable. In any case, the formation of an inorganic formate salt as coproduct is associated with disadvantages even when it can be utilized: first, the separation of the salt from TMP is complicated and incurs additional costs, while, second, the salt has to be worked up and purified if it is to be utilized in a beneficial fashion.
In an alternative process variant, the reaction of n-butyraldehyde with formaldehyde is carried out in the presence of a tertiary amine, usually a trialkylamine. However, the excesses of formaldehyde and trialkylamine used result in stoichiometric amounts of trialkylammonium formate being formed in addition to TMP. To improve the economics of such a process, it is necessary to recover the amine used from trialkyl-ammonium formate and preferably pass the formate to a purposeful use.
DE 25 07 461 A describes a process for preparing 2,2-dimethylol-alkanals that are converted into the corresponding trimethylol compound in a subsequent hydrogenation. Thus, for example, TMP is prepared by reaction of n-butyraldehyde with formaldehyde in the presence of catalytic amounts of tertiary amines and subsequent hydrogenation of the reaction product. A disadvantage of this process is that only unsatisfactory yields of trimethylolpropane are achieved.
DE 1 952 738 A describes a process for preparing TMP by reaction of n-butyraldehyde with formaldehyde in the presence of tertiary amines. The formate salts formed in the process are separated from TMP by distillation. It is proposed that the trialkylammonium formates thus formed be reacted with an aqueous calcium hydroxide solution to give calcium formate and liberate the amine, which is returned to the reaction circuit. A disadvantage of this process is that, once again, it results in formation of an organic formate salt that, if it is to be utilized further, has to be separated off and purified in a further reaction step. In addition, calcium hydroxide has to be used as additional starting material in order to convert the formate into a usable product.
In EP 142 090 A, TMP is prepared by reacting one mol of n-butyr-aldehyde with from 2.2 to 4.5 mol of formaldehyde and from 0.6 to 3 mol of trialkylamine and catalytically hydrogenating the resulting 2,2-dimethylol-butanal. A disadvantage is that the high amine concentrations in the aldol reaction result in formation of considerable amounts of trialkylammonium formates that have to be separated off by distillation prior to the hydrogenation. Liberation of trialkylamine from the formates formed and recirculation of the base to the process is not described.
In the method of reducing the amount of formate formed described in DE 28 13 201 A, formaldehyde is used in excess but the amine is used only as catalyst for the aldol reaction to form 2,2-dimethyloIbutanal. The aldehyde formed is subsequently catalytically hydrogenated. The process is not very suitable for industrial use since the excess of formaldehyde has to be separated off prior to the hydrogenation because of possible poisoning of the hydrogenation catalyst.
EP 289 921 A describes a process for preparing trimethylolalkanes which is similar to that described in EP 142 090 A and in which 1 mol of aldehyde is reacted with from 2.2 to 4.5 mol of formaldehyde in aqueous solution in the presence of from 0.6 to 3 mol of trialkylamine and the product is subsequently hydrogenated. To work up the trialkylammonium formate obtained, two process variants are reported. In variant (a), the crude hydrogenation mixture is heated to 100-200xc2x0 C. and water and excess trialkylamine are separated off by distillation. The trialkyl-ammonium formate remaining in the bottoms reacts with the alcohol present to give trimethylolalkane formate, thus liberating the amine used. The trimethylolalkane formate is subsequently transesterified with methanol to give methyl formate and trimethylolalkane. In process variant (b), the hydrogenation product is first substantially dewatered and the trialkylammonium formate remaining in the bottoms is subsequently esterified directly with methanol to form methyl formate. A disadvantage of both variants is that a loss of formaldehyde due to catalytic hydrogenation has to be accepted in order to achieve economical yields.
In DE 195 42 036 A, the recirculation of the tertiary amine used as base is carried out via esterification of the trialkylammonium formate formed with polymethylolalkane. A disadvantage of this method is that the ester formed has to be transesterified with a further, lower-boiling alcohol in a further step in order to liberate the polymethylolalkane.
WO 98/28253 A describes a process for preparing TMP without producing a coproduct. Here, n-butyraldehyde is reacted with from 2 to 8 times its molar amount of formaldehyde in a first reaction step in the presence of a tertiary amine as catalyst. The reaction mixture obtained is fractionally distilled in a second stage, where the distillate stream consisting predominantly of unreacted or partially reacted starting materials is returned to the first stage and the bottoms comprising predominantly 2,2-dimethylolalkanal are separated off or the reaction mixture from the first stage is separated by phase separation into an aqueous phase and an organic phase and the organic phase is returned to the first stage. In a third after-reaction stage, the bottoms fraction that has been separated off in the second stage or the aqueous phase obtained by phase separation in the second stage is subjected to a catalytic and/or thermal treatment in which the incompletely reacted compounds are converted into 2,2-dimethyolalkanal and starting materials that are returned to the first stage. Subsequently, 2,2-dimethylolalkanal is hydrogenated in a known manner to produce the corresponding trimethylol compound. However, this process has the disadvantage that the mono-methylolalkanal formed has to be eliminated from the reaction mixture by complicated measures since otherwise relatively large amounts of by-products are formed in the catalytic hydrogenation.
A further process for recovering the amines used is disclosed in DE 198 48 568 A and DE 198 48 569 A. After reaction of the aldehyde with aqueous formaldehyde in the presence of a tertiary amine, the reaction mixture is first freed of free amine and water by distillation. The trialkylammonium formate remaining in the bottoms is concentrated by distillation at a pH of 5 until trimethylolalkane formate and free amine are formed, with the latter being separated off as distillate. The trimethylol-alkane formate is decomposed catalytically under pressure at temperatures of about 280xc2x0 C. into trimethylolalkane, hydrogen, carbon dioxide, water, and carbon monoxide. A disadvantage of this process is that at least 1 mol equivalent of formaldehyde per mole of alkanal is not utilized economically.
It is therefore an object of the invention to provide a process for preparing trimethylol compounds in the presence of tertiary amines as base that allows the formate salts obtained to be converted into a usable form with recirculation of the amine used.
We have now surprisingly found a process for preparing trimethylol compounds and formic acid comprising
(a) reacting formaldehyde and an aldehyde in the presence of a nitrogen base to form a product mixture containing the trimethylol compound and a formate salt of the nitrogen base,
(b) removing the trimethylol compound from the product mixture after the reaction, and
(c) cleaving the formate salt into the free nitrogen base and formic acid by distillation in the presence of an auxiliary.